The assembly and population of a printed circuit board requires many placement, soldering, and other process steps. Therefore testing and inspection are vital to the econmical fabrication of a high quality product. This is especially true as chip circuit densities, I/O densities, and surface mount technologies drive up printed circuit board circuit loadings. Faults can have their origin in the component chips and bare boards, in component insertion, or in soldering. The faults themselves can be wrong values or labels, poor circuit performance, open circuits, short circuits, components in the wrong position, physical damage, improper solder, damaged or open lands, or out of tolerance faults.
Faults in surface mount technology packages frequently occur because the components are held in place by solder paste, glues, or temporary adhesives until solder reflow. During the interval between placement and reflow, lateral movement of as little as 0.025 inch and rotation of as little as 2.5.degree. from true orthogonal can both have catastrophic effects. Moreover, in surface mount technology, where a good solder joint requires that the solder joint fillet have a meniscus covering both the surface pad and the IC chip pad or lead, the presence of pad oxidation, pad contamination, or raised leads can all give rise to bad solder joints.
Faults in wave soldered plated through hole mounted components can occur during wave soldering. These faults can result in interpin shorts. In wave soldered plated through hole mount components a good soldered joint is charatcetrized by solder fillet having a meniscus that substantially covers the copper land and the plated through hole. Problems can arise through solder shortage, solder excess, voids, blowholes, and no solder.
one way of testing for defects is with contact testers, such as short testers, manufacturing defect analyzers, functional testers, in-circuit testers, and combination testers. Contact testers operate through the use of a mechanical system that pulls the populated printed circuit board down onto the probes so that the individual probes pins act as a bed of nails, contacting all of the test points on the printed circuit board. Some contact fixtures have as many as two thousand or three thousand or more probe pins.
Current test fixture fabrication practice involves the construction of a box which forms the framework of the fixture. Two plates are contained in the box, a stationary plate and a vertically movable plate, parallel to the stationary plate. The stationary plate, called the probe plate, contains the many probe pins. These probe pins contact the circuit under test. The test probes or probe pins are spring loaded devises which make contact with the device under test.
The spring loaded probe pin is installed in a socket which is positioned in the probe plate by holes which are drilled to correspond to the test point pattern of the board to be tested.
The top movable plate supports the board to be tested, which is located by tooling pins. This top plate has clearance holes which allow the test probes to pass through it and contact test locations on the board under test.
In this approach the additive nature of the tolerances involved in both fixture fabrication and board fabrication limit the minimum test target or contact size to about 0.030 inch diameter. Tolerance accumulations occur on the boards itself in drilling of board tooling holes and circuit via holes, as well as circuit pattern and component placement artwork registration. Tolerance is accumulated in the test fixture locating pin size and position, test probe locational accuracy, in addition to probe insertion angularity and pointing accuracy.